Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals or “qubits” transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the qubits will introduce errors and reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992).
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. During the QKD process, Alice uses a random number generator (RNG) to generate a random bit for the basis (“basis bit”) and a random bit for the key (“key bit”) to create a qubit (e.g., using polarization or phase encoding) and sends this qubit to Bob.
The above mentioned references by Bennett each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The interferometers need to be actively stabilized to within a portion of quantum signal wavelength during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
In the two-way system of the '234 patent, Alice includes an optical phase modulator and a Faraday mirror. The phase modulator is provided with a modulation randomly selected from a set of modulations. The modulation is timed to coincide with the arrival of one of two optical pulses from Bob. The pulses are then sent back to Bob, with one of the pulses having been modulated. The remaining pulse is likewise modulated at Bob. The pulses are interfered, and the resulting interfered pulse is detected. This process is repeated, and the usual QKD protocols and procedures are followed to establish a secure key between Alice and Bob.
It is imperative that a potential eavesdropper (Eve) not be able to discern the activity of Alice's phase modulator. If an eavesdropper were to know the state of Alice's modulator, she would be able to deduce the value of the exchanged pulses (qubits).
Alice's modulator activity is of interest to the QKD system only when qubits are actively being modulated. At times when there are no qubits in the vicinity of Alice, the modulator's value is of no interest because there is nothing to modulate. Consequently, present-day QKD systems leave the modulator at rest when qubits are not present. However, this makes the eavesdropping task for Eve considerably easier because she can focus her concentration on changes in the state of the modulator. If Alice's modulator is active only when it is modulating qubits, then if Eve has electromagnetic interference (EMI) measurement capability or probe-beam capability, there is only a relatively small amount of information that she needs to examine to determine how the qubits were modulated.